Plastic lead frames for semiconductor devices

ABSTRACT

A conductive plastic lead frame and method of manufacturing, the same suitable for use in IC packaging. In a preferred embodiment, the lead frame is constructed of a plastic or polymer based lead frame structure with an intrinsic conductive polymer coating. In a second embodiment, the lead frame is a composite plastic or polymeric material intermixed with an intrinsic conductive polymer coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/429,392,filed Oct. 28, 1999, now U.S. Pat. No. 6,316,824, issued Nov. 13, 2001,which is a divisional of application Ser. No. 09/193,469, filed Nov. 17,1998, now U.S. Pat. No. 6,091,136, issued Jul. 18, 2000, which is adivisional of application Ser. No. 08/878,935, filed Jun. 19, 1997, nowU.S. Pat. No. 5,879,965, issued Mar. 9, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit lead frames andmethods of production thereof. In particular, this invention relates toplastic lead frames with a conductive coating or material containedtherein used for packaging integrated circuits and methods ofmanufacturing the same.

2. State of the Art

Integrated circuit (IC) chips are enclosed in plastic packages thatprovide protection from hostile environments and enable electricalinterconnection to printed circuit boards. During a manufacturingprocess, the IC chip is typically attached to a die paddle of aconventional lead frame or suspended from the lead fingers of aleads-over-chip (LOC) lead frame using an adhesive such as epoxy ordouble-sided tape, and subsequently encapsulated with a dense and rigidplastic by a transfer molding process. In essence, the lead frame formsthe backbone of the molded plastic IC package.

Lead frames typically perform many functions such as: (1) a holdingfixture that indexes with tool-transfer mechanisms as the packageproceeds through various assembly operations, (2) a dam that preventsplastic from rushing out between leads during the molding operation, (3)a chip attach substrate, (4) a support matrix for the plastic, and (5)an electrical and thermal conductor from chip to board.

Traditionally, lead frames are fabricated from a strip of sheet metal bystamping or chemical milling operations. There are many different metalalloy compositions which are commercially available for producing leadframes. For example, Rao R. Tummala and Eugene J. Rymaszewski,“Microelectronics Packaging Handbook,” Table 8-4, 1989, provide 16different alloys available from 9 different manufacturers. Lead framematerial selection depends on many factors such as cost, ease offabrication, strength, thermal conductivity, and matched coefficient ofthermal expansion (CTE). A close match of CTE between the silicon dieand the lead frame is required to avoid chip fracture from differentexpansion rates.

The most widely used metal for lead frame fabrication is Alloy 42 (42%Nickel −58% Iron). Alloy 42 has a CTE near silicon and good tensilestrength properties. The disadvantage of Alloy 42 is that it has lowthermal conductivity. Since the lead frame is the main conduit by whichheat flows from the chip to the environment and printed circuit board,this can have a profound effect on the package thermal resistance afterprolonged device operation.

A layered composite strip, such as copper-clad stainless steel, wasdeveloped to emulate the mechanical properties of Alloy 42 whileincreasing thermal conductivity. However, copper-clad stainless steel issomewhat more expensive to manufacture than Alloy 42. When manufacturingcopper-clad stainless steel lead frames, the cladding is accomplished byhigh-pressure rolling of copper foil onto a stainless steel strip,followed by annealing the composite to form a solid-solution weld. Whilecopper alloys provide good thermal conductivity and have a CTE near thatof low-stress molding compounds, there is a substantial CTE mismatchwith respect to silicon.

While numerous alloys have been developed to solve problems with thermalconductivity, CTE mismatch, and strength, other important factors, suchas ease of fabrication and cost, have not improved as readily.

Conventional methods for making lead frames for integrated circuitdevices are described in U.S. Pat. No. 3,440,027. The use of a plasticsupport structure in a method of forming metal lead frames is describedin U.S. Pat. No. 4,089,733 (hereinafter “the '733 patent”). The plasticsupport structure of the '733 patent solves the problem of deformed andmisaligned lead fingers resulting from stress during the bonding processby supporting the lead fingers with a plastic structure. However, the'733 patent requires a metal lead frame in addition to the plasticsupport structure with its attendant costs. A method of manufacturingmultilayer metal lead frames is disclosed in U.S. Pat. No. 5,231,756(hereinafter “the'756 patent”). The '756 patent provides an improvementin aligning power and ground planes for use in a multilayer lead framewhere such planes are necessary. However, the number of steps requiredto manufacture such multilayer lead frames will not solve the problem ofdecreasing costs. In short, none of the related art appears to disclosemethods of producing low-cost lead frames made from materials notstructurally based on metal.

Since packaged ICs are produced in high volumes, a small decrease in thecost per packaged IC can result in substantial savings overall.Accordingly, there is a need in the industry for a low-cost plastic leadframe with suitable characteristics for IC packaging.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises plastic lead frames coated withconductive materials or having conductive materials therein suitable foruse in IC packaging and methods for fabricating the same. The inventionmay be used in the production of ICs.

By using plastic as the structural base for a lead frame, many costsassociated with the manufacture of metal lead frames can be eliminated.For instance, plastic lead frames can be injection molded or stamped andthen coated with an intrinsic conductive polymer. Furthermore, plasticis intrinsically less expensive as a bulk material than metal alloystypically used in lead frame construction.

Once the lead frame is formed, it can be used in either the conventionaldie attach and connect process or in a LOC process. In the conventionalprocess, the die is adhesively attached to a die paddle using epoxy ordouble-sided tape, followed by wire bonding where die pads are connectedto lead fingers from the lead frame. In a LOC process, the die could beattached to the lead frame fingers by double-sided adhesive tapefollowed by wire bonding. Alternatively, the LOC process might includedirect connection between the die pads and the LOC lead frame fingerswith a conductive epoxy or Z-axis conductive material using methodscommon in the art.

In the preferred embodiment of the invention, a plastic lead framecoated with an intrinsically conductive polymer is provided. The plasticlead frame structure can be formed by injection molding, stamping oretching from a sheet of plastic or polymer material. This plastic leadframe structure is then coated with a conductive polymer by dipping in asolution or lacquer composed of a polyaniline such as the commerciallyavailable product Ormecon™. By controlling the polyaniline coatingprocess, precise layers with known thicknesses can be produced. Theresulting low-cost lead frame has a conductive layer surrounding theplastic structure. Moreover, the CTE of silicon, conductive polymer, andmolding compounds is very nearly matched.

Another embodiment of the present invention is a composite plastic leadframe formed of a conventional polymer intermixed with a conductivepolymer. The composite plastic lead frame structure is formed byinjection molding, stamping or etching from a sheet of the compositeplastic/conductive polymer material. The CTE of the lead frame is wellmatched to that of silicon as in the preferred embodiment; however,increased quantities of polyaniline required to provide sufficientconductivity may consequently increase cost relative to the preferredembodiment.

Additional advantages of both the above embodiments are transparency,corrosion resistance, and oxidation resistance. Polyaniline istransparent. By using transparent plastic or polymer in the lead framestructure, ultraviolet (UV), or other light source, cure of the dieattach material becomes possible. This is particularly advantageous inan automated production environment. Furthermore, both of the above leadframe embodiments are nonmetallic and thus less susceptible to corrosionor oxidation.

The inventive plastic lead frames solve the problem of reducing costwhile maintaining characteristics necessary for use in commercialproduction of IC packages. The overall cost of IC chip packaging isreduced by using plastic lead frames coated with conductive layers. Useof transparent polymers and intrinsically conductive polymersfacilitates UV or other light source cure of die attach materials.Furthermore, the methods used to produce such lead frames are simple andcan be easily incorporated into existing high-speed production lines formanufacturing IC chips. While the inventive plastic lead framesdescribed herein have been illustrated with respect to conventional wirebonding and LOC interconnect technology, there is theoretically nolimitation to applying the invention to conventional lead frames,emerging tape automated bonding (TAB) technology, etc. as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of the inventive lead frame in a LOCconfiguration with direct attachment to IC die pads;

FIG. 2 is a blowup cross-sectional view of the inventive lead frame asit would be attached to an input/output pad of an IC die in the LOCconfiguration;

FIG. 3 is a perspective view of the inventive lead frame in a LOCconfiguration where the IC die is adhesively attached to the leadfingers and wire bonded to die bond pads;

FIG. 4 is a cross-sectional view of the preferred embodiment showing theplastic frame structure with a coating of polyaniline of thickness “d”;

FIG. 5 is a perspective view of the inventive lead frame in aconventional lead frame configuration with wire bond attachment from ICbond pads to the lead fingers of the lead frame;

FIG. 6 is a cross-sectional view of an embodiment of the presentinvention of a LOC type lead frame in an encapsulated package mounted ona substrate; and

FIG. 7 is a cross-sectional view of another embodiment of the presentinvention of a conventional type lead frame in an encapsulated packagemounted on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

An understanding of the detailed description of the invention isfacilitated by reference to the drawings, FIGS. 1 through 7. Each of thefour embodiments of the invention solves the problem of reducing cost ofproducing lead frames for IC chip packaging. Additionally, at least twoof the embodiments improve the following characteristics: CTE matchingof the lead frame, silicon, and adhesive, anti-corrosion,anti-oxidation, and in-line cure of the die attach adhesive.

Drawing FIG. 1 shows the preferred embodiment of the inventive plasticlead frame 10 as envisioned for application in a LOC packagingconfiguration. The lead fingers 12 of the plastic lead frame 10 (notcompletely shown) are positioned over die bond pads 14. The lead fingers12 are directly connected to the die bond pads 14 by an adhesive 16consisting of a conductive epoxy or Z-axis conductive material. The ICchip or die 18 is suspended by the adhesive 16 connecting the leadfingers 12 of the plastic lead frame 10.

Drawing FIG. 2 depicts an enlarged cross-sectional view of the preferredembodiment showing a lead finger 12 as attached to a die bond pad 14 onthe IC die 18. The cross-section of the lead finger 12 is also shownwith the inner lead frame structure 20 coated with a conductive coating22. The conductive coating 22 could be a conductive epoxy, Z-axisconductive material, or any other suitably conductive adhesive known inthe art. The die bond pad 14 is connected to a circuit trace 24 leadingto components (not shown) on the IC die 18. The circuit trace 24 willtypically be underneath a passivation layer of oxide 26 on the IC die18.

Drawing FIG. 3 shows a perspective view of the preferred embodiment in aLOC configuration which utilizes conventional wire bonding. In drawingFIG. 3, lead fingers 12 of the plastic lead frame 10 (not completelyshown) are located over an adhesive tape 28 which holds the IC die 18 tothe plastic lead frame 10. Die bond pads 14 are connected to leadfingers 12 by means of wires 30. The wires 30 can be aluminum or goldand are attached using wire bonding machines (not shown) wellestablished in the art.

Drawing FIG. 4 shows a magnified cross-section of a plastic lead finger12 of the preferred embodiment of the inventive plastic lead frame 10(not shown). The inner plastic lead frame structure 20 is made of aconventional plastic or polymer material. The surrounding conductivecoating 22 is an intrinsic conductive polymer, such as polyaniline, orcopper. The polyaniline layer is of thickness “d.” The minimum thickness“d” necessary for suitable electrical conductivity is governed by thefollowing equation:

d 1/(πfσμ)^(1/2)

where f is the maximum frequency of the electrical device, μ is thepermeability of the polyaniline layer, and σ is the conductivity of thepolyaniline layer. For example, where f is 1×10⁹ Hz, σ is 1×10⁵(Ohm·m)⁻¹ and μ is 1.26×10⁻⁶ Henry/m, a thickness of 50 μm is needed forthe polyaniline coating.

Drawing FIG. 5 shows a perspective view of the preferred embodiment in aconventional type lead frame configuration which utilizes conventionalwire bonding. In drawing FIG. 5, lead fingers 112 of the plastic leadframe 100 (not completely shown) are located adjacent the sides 116which hold the IC die 118 to the lead frame 100. Die bond pads 114 areconnected to lead fingers 112 by means of wires 130. The wires 130 canbe aluminum or gold and are attached using wire bonding machines (notshown) well established in the art. The IC die 118 is supported by thedie paddle 120 of the lead frame 100 and is adhesively secured theretoby means of a suitable epoxy adhesive or, alternately, by means of adouble-sided adhesively coated tape.

Drawing FIG. 6 shows an IC die 218 encapsulated by material 230connected to a LOC type lead frame 200 having lead fingers 212 connectedto the bond pads on the active surface of the IC die 218 and connectedto electrical circuits (not shown) on a substrate 220, such as a printedcircuit board or the like. The lead fingers 212 may be shaped in anysuitable type configuration for connection to the IC die 218 and theelectrical circuits of substrate 220. The encapsulating material 230 maybe of any well known suitable type and may include suitable fillermaterial therein.

Drawing FIG. 7 shows an IC die 318 encapsulated by material 340connected to a conventional type lead frame 300 having lead fingers 312and a die paddle 316 supporting the IC die 318. The lead fingers 312 areconnected to the bond pads 314 on the active surface of the IC die 318by wires 330 and are connected to electrical circuits (not shown) on asubstrate 320. The lead fingers 312 may be shaped in any suitable typeconfiguration for connection to the IC die 318 and the electricalcircuits of substrate 320. The encapsulating material 340 may be anywell known suitable type and may include suitable filler materialtherein.

In the preferred embodiment, plastic lead frames can be dipped in anintrinsically conductive polymer, such as polyaniline, to form aconductive layer. The polyaniline dip could be a dispersion ofpolyaniline in a mixture of organic solvents. The coating is finished bydrying the coating with infrared heating or baking using techniques wellknown in the art. This dip coating process can be repeated as necessaryto attain the desired thickness. Alternatively, the polyaniline can beapplied in a lacquer dispersion, again using techniques well known inthe art.

In a second embodiment, the polyaniline could be dispersed throughoutthe plastic lead frame structure using commercially availablepolyaniline polymer dispersions such as those offered by ZipperlingKessler & Co. The composite lead frame could be formed by injectionmolding or stamping a sheet of the composite polymer containingpolyaniline.

In both the preferred and second embodiment, such plastic lead framesdescribed have less CTE mismatch with respect to the silicon IC die,plastic mold compounds, and die attach materials relative to traditionalmetal alloys. Furthermore, conductive plastic lead frames are moreflexible than metal alloy lead frames to resist bent leads. Theinventive plastic lead frames can also be made transparent to enableultraviolet radiation cure (or other light source cure) of die attachmaterials. This is a distinct advantage over metal alloys which are nottransparent.

In a third embodiment, the plastic lead frame is formed from aninjection molded or stamped plastic or polymer based sheet of materialto form a lead frame structure. The plastic lead frame structure wouldthen be coated with copper, using electroless copper plating techniqueswell-known in the industry. Alternatively, the plastic lead framestructure could be coated with copper using chemical vapor deposition orother plating techniques known in the art. After the copper coating hasbeen applied to the plastic lead frame, the copper may have one or morecoatings or layers of coatings of other conductive metal thereon, suchas a layer of nickel, palladium, silver, gold, other precious metals,etc. In this third embodiment, the resulting low-cost plastic lead framehas a conductive layer of copper with suitable electrical and thermalcharacteristics for IC packaging. However, the CTE mismatch between thecopper plated plastic lead frame and the silicon IC die should becomparable to that associated with traditional copper-clad lead frames.

Plastic lead frames can be manufactured by injection molding,compression molding or stamping to form complex and intricate shapes.There is no particular limitation on the lead frame shape complexityother than the tools used to perform the injection molding or stamping.Furthermore, by reducing the number of steps necessary to produce theplastic lead frame, relative to a metal lead frame, a lower cost can beachieved.

Although the present invention has been described with reference toparticular embodiments, the invention is not limited to these describedembodiments. Rather, the invention is limited only by the appendedclaims, which include within their scope all equivalent devices ormethods which operate according to the principles of the invention asdescribed.

What is claimed is:
 1. An electronic device comprising: an integratedcircuit semiconductor die having at least one bond pad thereon forattachment in one of a conventional die attach to lead frame connectprocess and a LOC connect process; a conductive plastic lead framehaving a plurality of lead fingers, the conductive plastic lead frameincluding a plastic lead frame structure having a conductive polymericmaterial located on at least a portion of the plurality of lead fingersand formed by one of compression molding and injection molding, aportion of each of the plurality of lead fingers formed for contacting acircuit of the circuits of the substrate; an adhesive attaching aportion of the integrated circuit semiconductor die to a portion of theconductive plastic lead frame; at least one connector connecting the atleast one bond pad of the integrated circuit semiconductor die to atleast one lead finger of the plurality of lead fingers of the conductiveplastic lead frame; and encapsulating material for encapsulating atleast a portion of the integrated circuit semiconductor die and forencapsulating at least a portion of the at least one lead finger of theconductive plastic lead frame.
 2. An electronic device comprising: anintegrated circuit semiconductor die having at least one bond padthereon for attachment in one of a conventional die attach to lead frameconnect process and a LOC connect process; a conductive plastic leadframe having a plurality of lead fingers, the conductive plastic leadframe including a plastic lead frame structure having a conductivepolymeric material located on at least a portion of the plurality oflead fingers, a portion of each of the plurality of lead fingers formedfor contacting a circuit of the circuits of the substrate; an adhesivefor attaching a portion of the integrated circuit semiconductor die to aportion of the conductive plastic lead frame; at least one connectorconnecting the at least one bond pad of the integrated circuitsemiconductor die to at least one lead finger of the conductive plasticlead frame; and encapsulating material for encapsulating at least aportion of the integrated circuit semiconductor die and forencapsulating at least a portion of the at least one lead finger of theconductive plastic lead frame.